Welding method for disposable diapers

ABSTRACT

A welding method for a disposable diaper includes the steps of putting wing-like portions 21, 22 provided on front and rear bodies 5, 6 including a topsheet 2, a backsheet 3 and a core 4 sandwiched therebetween, one upon another, with the topsheet disposed face to face; placing covering sheets 20 on an outer surface of the wing-like portions, both the covering sheets and the wing-like portions having a melting point higher than the melting point of the backsheet; and welding, under heat and pressure exerted from above the covering sheets, laterally opposite side edges of the wing-like portions.

This application is a division of application Ser. No. 08/242,975 filedMay 16, 1994.

BACKGROUND OF THE INVENTION

The present invention relates to a method of welding a disposable diaperalong laterally opposite side edges at its waist level.

One example of a pant-type of disposable diapers is disclosed inJapanese patent application Disclosure No. 5-15551. In that reference, adiaper comprises top- and backsheets and a liquid-absorbent coresandwiched therebetween. The diaper is folded along a transverse centerline passing through a crotch zone so that front and rear bodies arepositioned one upon another. The top- and backsheets are ultrasonicallywelded together along laterally opposite side edges at its waist levelto obtain a pant-type of a configuration. The top- and backsheets may beformed, for example, by nonwoven fabric of thermoplastic synthetic resinwhich can be ultrasonically heated.

Nonwoven fabric employed in the technique disclosed in said Disclosureas material for the top- and backsheets may be made from fibers of thesame type as mentioned above and heated until the nonwoven fabricexhibits an appropriate viscosity so that a weld zone of the top- andbacksheets presents a smoothly finished appearance. However, if thetopsheet formed of nonwoven fabric of polypropylene fiber having arelatively high melting point is combined with the backsheet formed ofpolyethylene sheet or film having a relatively low melting point, thepolyethylene sheet will melt earlier than the polypropylene fiber whenthe diaper is folded and subjected to the ultrasonic treatment. Inconsequence, the polyethylene sheet may melt to an excessively lowviscosity and stick to a horn pressed against this sheet during theultrasonic treatment. The polyethylene sheet which has stuck to the hornwill, once cooled, result in a rough, itchy and uncomfortable surfaceand spoil the aesthetical appearance of the diaper. If the polyethylenesheet remains stuck to the horn, no accurate clearance can be assuredbetween the horn and an anvil during the next cycle of the ultrasonictreatment, thus making the continuous treatment difficult.

Accordingly, it is a principal object of the invention to solve theaforementioned problem by placing a covering sheet having a meltingpoint higher than a melting point of a backsheet upon an outer surfaceof the backsheet and bringing it in contact with the pressing means suchas a horn for an ultrasonic treatment.

SUMMARY OF THE INVENTION

The object set forth above is achieved, according to the invention, by awelding method for a disposable diaper generally comprising the steps ofassembling a liquid-permeable topsheet, a liquid-impermeable backsheetand a liquid-absorbent core sandwiched between these sheets into aconfiguration of disposable diaper, putting wing-like portions of frontand rear bodies outwardly extending from laterally opposite side edgesof the core one upon another and welding the wing-like portions underheat and pressure together to form opposite side portions at waistlevels of the front and rear bodies. The welding method comprises thesteps of assembling topsheet members associated with the topsheet so asto form extensions thereof with the backsheet made of material having amelting point lower than a melting point of the topsheet members to formthe wing-like portions, putting the wing-like portions one upon anotherwith the topsheet members disposed face to face inside the diaper,placing covering sheets having a melting point higher than the meltingpoint of the backsheet on an outer surface of the backsheet, and weldingthe wing-like portions along laterally opposite side edges thereoftogether with the covering sheets.

Preferably, the melting point of the covering sheets is higher than themelting point of the topsheet members.

According to the welding method as described above, the heating/pressingmeans such as an ultrasonic horn or a heating element of desired shapeis pressed against the sheet portions to be welded together. Thecovering sheet having a melting point higher than that of the backsheetis brought in contact with the heating/pressing means so as to preventthe molten backsheet from sticking to the heating/pressing means.

BRIEF DESCRIPTION OF THE DRAWINGS

A welding method of the invention will be described in more detail withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a pant type disposable diaper;

FIG. 2 is a plan view of the diaper as unfolded;

FIG. 3 is a sectional view along a line X--X in FIG. 2;

FIG. 4 is a schematic diagram illustrating ultrasonic treatment;

FIGS. 5A and 5B are sectional views of weld zones presenting differentconfigurations; and

FIGS. 6A and 6B are views similar to FIG. 3 showing differentconfigurations.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 is a perspective view showing a pant-type of disposable diaper 1obtained by a welding method of the invention. The diaper 1 comprises aliquid-permeable topsheet 2 formed of a melt bond nonwoven fabric ofpolypropylene fiber, a liquid-impermeable backsheet 3 formed of apolyethylene sheet and a liquid-absorbent core 4 sandwiched betweenthese sheets 2, 3. The diaper 1 is also generally configured by a frontbody 5, a rear body 6 and a crotch zone 7. A waist-opening 11 andrespective leg-openings 12 are provided with elastically stretchablemembers 13, 14, respectively, which are adhesively attached theretoalong their peripheries. The front and rear bodies 5, 6 are laid one ontop of another with the topsheet 2 disposed face to face inside thediaper 1 and the backsheet 3 is provided along laterally opposite sidesof waist portions of the front and rear bodies 5, 6 with covering sheets20 so that these sheets 2, 3, 20 are welded together by an ultrasonicintermittent weld line (see reference numeral 15) vertically extendingalong lateral sides of the waist portions.

FIG. 2 is a plan view of the diaper 1A prior to the welding depictedunfolded longitudinally and partially broken away. In the diaper 1A, thewaist portions of the front and rear bodies 5, 6 extend outwardly fromlateral side edges of the liquid-absorbent core 4 to define wing-likeportions 21, 22, respectively. It should be understood that thesewing-like portions 21, 22 may be defined by topsheet members and/orbacksheet members of the desired characteristics bonded to the topsheetand/or the backsheet to form extensions thereof rather than the top- andbacksheets. The wing-like portions 21, 22 carry on their side of thebacksheet 3 the previously mentioned polypropylene covering sheets 20.On laterally opposite sides of the diaper 1A inside the wing-likeportions 21, 22, there are provided a pair of flaps 31 longitudinallyextending on the topsheet 2. Each flap 31 is bonded to the topsheet 2along its outer edge 25 and its longitudinally opposite ends 26, 27 sothat its inner edge 28 may rise from the topsheet 2 under the effect ofan elastic member 29 bonded, in a stretched condition, to the inner edge28.

FIG. 3 is a sectional view along a line X--X in FIG. 2. It should beunderstood here that a sectional view along a line Y--Y in FIG. 2 issubstantially the same as FIG. 3. Over each wing-like portion 21 of thediaper 1A, the top- and backsheets 2, 3 are intermittently bonded toeach other by means of hot melt adhesive 33, and the backsheet 3 and thecovering sheet 20 are also bonded to each other by means of hot meltadhesive 33. Each flap 31 cooperates with the topsheet 2 to form apocket 30 opening inwardly of the diaper 1A. The liquid-absorbent core 4may be bonded, if necessary, to the topsheet 2 and/or the backsheet 3.

FIG. 4 is a schematic side view illustrating a manner in which saiddiaper 1A is folded inwardly along a center line C--C in FIG. 2 and thewing-like portions 21, 22 of the front and rear bodies overlappedtogether are welded together utilizing an ultrasonic welder 40. Thewelder 40 may be of the conventional type and comprises a horn 41connected to an ultrasonic oscillator (not shown) and a stationary anvil42, between which the wing-like portions 21, 22 are held and weldedtogether so as to form a weld zone 15 (FIG. 1) conformable to the shapeof the horn 41 at its forward end.

In the ultrasonic treatment of the diaper 1A, the optimal condition ofthe treatment must be selected to weld together sections of the topsheet2 folded against each other. During this treatment, the polyethylenesheet forming the backsheet 3 having a melting point lower than amelting point of the polypropylene sheet forming the topsheet 2 isheated earlier to become sticky and, in order to prevent thepolyethylene sheet (backsheet 3) from sticking to the horn 41, thepolypropylene covering sheet 20 is interposed between the polyethylenesheet (backsheet 3) and the horn 41.

FIGS. 5A and 5B are is a schematic side views of the weld zones obtainedby the ultrasonic treatment, wherein FIG. 5A illustrates the weld zone15 obtained utilizing the covering sheet 20 according to the method ofthe invention and FIG. 5B illustrates the case in which no coveringsheet 20 is utilized. In the case illustrated by FIG. 5A, the decreaseof viscosity of the polypropylene sheet (backsheet 3) is not significantand the welding occurs in a rather viscous molten state, so a mark madeby the horn 41 on the weld zone 15 is round, resulting in a smoothfinish which does not irritate the wearer's skin. In the caseillustrated in FIG. 5B, on the other hand, the polyethylene sheet(backsheet 3) is molten and sticks to both the horn 41 and the anvil 42when the molten state of the polypropylene sheet (topsheet 2) is of arelatively high viscosity, so the weld zone 15a after cooled presents arough surface giving an itchy stimulus to the wearer's skin.

FIGS. 6A and 6B are sectional views showing configurations differentfrom each other and different from the configuration shown by FIG. 3. InFIG. 6A, portions of the topsheet 2 extending from the laterallyopposite side edges of the core 4 are extremely small. However, outerzone 25 of each flaps 31 bonded to the topsheet 2 by means of hot meltadhesive 33 is a relatively large and cooperates with the backsheet 3 tofrom the wing-like portion 21. The front and rear bodies 5, 6 (see FIG.2) are folded along a boundary line therebetween to put the outer zone25 on the side of the front body 5 and the outer zone 25 on the side ofthe rear body 6 together and these outer zones 25 are subjected to theultrasonic treatment. Each flap 31 is formed by spun bond nonwovenfabric of polypropylene. In FIG. 6B, a topside sheet member of eachwing-like portion 21 comprises spun bond nonwoven fabric covering sheet37 of polypropylene which covers the associated outer zone 25. Thetopsheet 2, the outer zone 25 and the covering sheet 37 are bondedtogether by means of hot melt adhesive 33. The front and rear bodies 11,12 are folded up against to each other so as to place the covering sheet37 on the side of the front body 11 and the covering sheet 37 on theside of the rear body 12 one on another. This area is then subjected tothe ultrasonic treatment. In both configurations shown by (A) and (B),the backsheet 3 is made of polyethylene.

It is also possible without departing from the scope of the invention toform each of the topside sheet member, the backsheet 3 and the coveringsheet 20 from a mixture of fibers having different melting points. Thedifferential melting point in each sheet should be understood here asthe differential melting point between the composing fibers totallyoccupying 60% or higher by weight of this sheet.

The melting point of the covering sheet 20 should be higher than themelting point of materials composing the backsheet 3 and, morepreferably, equal to or higher than the melting point of the topsidesheet member so that the formation of a desirably shallow and smallpress mark under the heating effect may be facilitated.

When nonwoven fabric composed of polypropylene is used to form thetopsheet 2 and the topside sheet member as in the diaper 1 illustratedas an embodiment of the invention, the polypropylene fiber having arelatively high firmness will make the nonwoven fabric cushiony,depending on a fineness as well as a density, and give the wearer acomfortable feel when the diaper 1 is warn. Use of the polyethylenesheet as the backsheet 3 will allow this relatively inexpensive sheet toprovide a comfortable soft touch.

It should be understood that the covering sheet 20 may be dimensioned soas to cover a desired area or whole area of the backsheet 3, having beenand previously laminated thereon.

According to the welding method of the invention, the covering sheethaving a melting point higher than that of the backsheet is interposedbetween the heating/pressing means such as the ultrasonic horn and thebacksheet in order to prevent the molten backsheet from sticking to theheating/pressing means. The present invention avoids the prior artproblem resulting, the weld zone after cooling, a rough surface whichmay give the wearer's skin an itchy stimulus and/or spoil an aestheticappearance of the diaper. Moreover, according to he present invention,the process of ultrasonic treatment is well stabilized, since no moltensheet material sticks to the horn.

What is claimed is:
 1. A welding method for a disposable diapergenerally comprising the steps of assembling a liquid-permeabletopsheet, a liquid-impermeable backsheet and a liquid-absorbent coresandwiched between these sheets into a configuration of a disposablediaper, putting wing-like portions of front and rear bodies outwardlyextending from laterally opposite side edges of said core one uponanother and welding said wing-like portions put one upon another underheat and pressure together to form opposite side portions at waistlevels of said front and rear bodies, said welding method furthercomprising the steps of:assembling topsheet members associated with saidtopsheet so as to form extensions thereof with said backsheet made ofmaterial having a melting point lower than a melting point of saidtopsheet members to form said wing-like portions, putting said wing-likeportions one upon another with said topsheet members disposed face toface inside said diaper, placing covering sheets having a melting pointhigher than the melting point of said backsheet on an outer surface ofsaid backsheet, and welding said wing-like portions along laterallyopposite side edges thereof together with said covering sheets.
 2. Themethod of claim 1, wherein the covering sheets are made ofpolypropylene.
 3. The method of claim 1, wherein the backsheet is madeof polyethylene and the covering sheets are made of polypropylene. 4.The method of claim 1, wherein the welding method produces weld zoneshaving a smooth finish.
 5. The method of claim 1, wherein the step ofputting the wing-like portions one upon another comprises the steps offorming the wing-like portions by bonding supplemental covering sheetsto the topsheet, wherein the step of assembling topsheet membersassociated with said topsheet so as to form extensions thereof with saidbacksheet comprises assembling said supplemental covering sheets.
 6. Themethod of claim 1, wherein prior to the welding step, the method furthercomprises the step of bonding a pair of flaps to the topsheet onlaterally opposite sides of the absorbent core inside the wing-likeportions.
 7. The method of claim 6, wherein extensions of the flaps formthe wing-like portions of the topsheet.
 8. The method of claim 1,wherein prior to the step of putting the wing-like portions one uponanother, the method further comprises the step of bonding a pair offlaps to the topsheet on laterally opposite sides of the absorbent core;and wherein the step of putting the wing-like portions one upon anothercomprises the steps of forming the wing-like portions by bondingsupplemental covering sheets to extensions of the flaps outwardly of theflaps, wherein the step of assembling topsheet members associated withsaid topsheet so as to form extensions thereof with said backsheetcomprises assembling said supplemental covering sheets.